Temperature compensated piezoelectric oscillator and electronic apparatus comprising it

ABSTRACT

A temperature-compensated piezoelectric oscillator includes an AT-cut quartz crystal resonator, an amplifying circuit connected to one end of the quartz crystal resonator, a varactor diode connected to the other end of the quartz crystal resonator, and a temperature compensation voltage generation circuit connected to ends of the varactor diode via resistors. The temperature compensation voltage generation circuit includes a first voltage generation circuit that includes thermistors and resistors and that is connected to the cathode of the varactor diode, and a second voltage generation circuit that includes a thermistor and resistors and that is connected to the anode of the varactor diode (VD).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to piezoelectric oscillators, and moreparticularly, to a temperature-compensated piezoelectric oscillator thatcompensates for an oscillation frequency in accordance with an ambienttemperature and also relates to an electronic apparatus including thetemperature-compensated piezoelectric oscillator.

2. Description of the Related Art

In general, piezoelectric oscillators include a piezoelectric element,such as a crystal strip, that resonates at a predetermined frequency inaccordance with an applied voltage and an amplifying circuit foramplifying a resonant signal by the piezoelectric element and foroutputting the amplified resonant signal. The resonant frequency of thepiezoelectric element, such as a crystal strip, is dependent upon thetemperature. Thus, even if the same voltage is applied, the resonantfrequency is changed as the temperature of the element changes.

In order to solve this problem, a plurality of temperature-compensatedpiezoelectric oscillators including a variable capacitance element, suchas a varactor diode, that is connected to a piezoelectric element and atemperature compensation voltage generation circuit for changing avoltage applied to the variable capacitance element in accordance withthe ambient temperature are known (for example, see Patent Document 1:Japanese Unexamined Patent Application Publication No. 2002-135053;Patent Document 2: Japanese Unexamined Patent Application PublicationNo. 2002-76773; and Patent Document 3: Japanese Unexamined PatentApplication Publication No. 6-224635).

In such temperature-compensated piezoelectric oscillators, a resonantfrequency depends on a combined capacitance of a piezoelectric elementand a variable capacitance element. Adjusting a voltage applied to thevariable capacitance element changes the capacitance of the variablecapacitance element. As a result, the combined capacitance is changed,and the resonant frequency is changed. By setting the amount of changein the resonant frequency to compensate for the amount of change in theresonant frequency caused by the temperature of the piezoelectricelement, a temperature-compensated piezoelectric oscillator that outputsa high-frequency signal having a constant resonant frequency withoutbeing affected by the ambient temperature is provided.

In each of the known temperature-compensated piezoelectric oscillators,an output voltage from a temperature compensation voltage generationcircuit is applied to one end of a variable capacitance element (forexample, a varactor diode), and the other end of the variablecapacitance element is grounded or set to a constant voltage.

Such a temperature-compensated piezoelectric oscillator is installed ina mobile communication apparatus or other suitable apparatus, and isused as a reference signal source. In recent years, a reduction involtage has been required for mobile communication apparatuses. Inaccordance with this reduction, a reduction in voltage has also beenrequired for temperature-compensated piezoelectric oscillators, whichare used as reference signal sources.

Known temperature compensation voltage generation circuits include athermistor, which is a thermo-sensitive element, as described in theabove-mentioned patent documents. Applying a low voltage to the circuitgenerates an output voltage corresponding to the temperature, and thevoltage is supplied to a variable capacitance element. Normally, due tosimplification of the circuit, a power supply voltage of thetemperature-compensated piezoelectric oscillator is used as a voltagesource for supplying the low voltage to the temperature compensationvoltage generation circuit.

Thus, as described above, in accordance with the reduction in thevoltage in the temperature-compensated piezoelectric oscillator, thevoltage supplied to the temperature compensation voltage generationcircuit is reduced. As a result, an output voltage, that is, the maximumvalue of the voltage supplied to the variable capacitance element isreduced. Thus, the range of the voltage applied to the variablecapacitance element is reduced, and the range of possible changes in thecapacitance is reduced.

In contrast, although the resonant frequency of a piezoelectric element,such as a quartz crystal resonator, is dependent upon a change in thetemperature, the resonant frequency does not depend on the appliedvoltage. Thus, even if the voltage of the temperature-compensatedpiezoelectric oscillator is reduced, the amount of change in theresonant frequency with respect to a change in the temperature does notchange.

Accordingly, a sufficient temperature compensation for the resonantfrequency of the piezoelectric element may not be achieved in the rangeof the voltage generated from the temperature compensation voltagegeneration circuit.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a temperature-compensated piezoelectricoscillator that ensures temperature compensation and that outputs ahigh-frequency signal having a constant resonant frequency even when apower supply voltage is reduced, and an electronic apparatus includingsuch a novel temperature-compensated piezoelectric oscillator.

According to a preferred embodiment of the present invention, in atemperature-compensated piezoelectric oscillator including apiezoelectric element, an amplifying circuit connected to one end of thepiezoelectric element, a variable capacitance element connected theother end of the piezoelectric element, and a compensation voltagegeneration circuit for applying a voltage corresponding to a temperatureto the variable capacitance element, the compensation voltage generationcircuit includes a first voltage generation circuit for applying a firstvoltage to one end of the variable capacitance element that is variabledepending upon an ambient temperature and second voltage generationcircuit for applying a second voltage to the other end of the variablecapacitance element that is variable depending upon the ambienttemperature in a direction opposite to the first voltage.

With this structure, a voltage that is variable depending upon theambient temperature and that is in accordance with a potentialdifference between the first voltage generated by the first voltagegeneration circuit and the second voltage generated by the secondvoltage generation circuit is applied to the variable capacitanceelement connected to the piezoelectric element. Thus, by setting therange of possible voltages generated by the first voltage generationcircuit to be different from the range of possible voltages generated bythe second voltage generation circuit, a voltage change depending uponthe temperature in a wider voltage range can be applied to the variablecapacitance element, as compared to a case where one end of the variablecapacitance element is set at a constant voltage. Accordingly, thecapacitance range of the variable capacitance element is increased, andthe capacitance changes depending upon the ambient temperature. As aresult, even if a power supply voltage is reduced, the capacitance rangeis not reduced, and the capacitance greatly changes depending upon thetemperature in the capacitance range. By setting the amount of change inthe capacitance caused by the temperature to correspond to the amount ofchange in the resonant frequency caused by the temperature of thepiezoelectric element, the resonant frequency of a resonant circuitincluding the piezoelectric element and the variable capacitance elementis compensated for.

Also, each of the first and second voltage generation circuitspreferably includes at least one thermo-sensitive element and aplurality of resistance elements.

The thermo-sensitive element is preferably a thermistor.

With this structure, each of the first and second voltage generationcircuits, which applies a voltage to the variable capacitance element,is defined by a simple analog network including the thermistor and theresistors.

Preferably, the temperature-compensated piezoelectric oscillator furtherincludes a temperature compensation data generation circuit fordetecting the ambient temperature and for generating temperaturecompensation data corresponding to the detected temperature. Each of thefirst and second voltage generation circuits includes a DA converter forconverting the temperature compensation data in a digital format into ananalog signal.

With this structure, the temperature compensation data generationcircuit stores temperature compensation data corresponding to a detectedtemperature in advance, and the temperature compensation datacorresponding to the detected temperature is output to each of the firstand second voltage generation circuits. Each of the first and secondvoltage generation circuits converts the temperature compensation datain the digital format into a voltage signal in an analog format, andapplies the voltage signal to the variable capacitance element. Thecapacitance of the variable capacitance element changes in accordancewith a potential difference between the voltage signal applied from thefirst voltage generation circuit and the voltage signal applied from thesecond voltage generation circuit. Since the temperature compensationdata corresponds to the amount of change in the resonant frequency dueto the temperature of the piezoelectric element, the resonant frequencyof the resonant circuit including the piezoelectric element and thevariable capacitance element is appropriately compensated for.

The piezoelectric element is preferably an AT-cut quartz crystalresonator.

The variable capacitance element is preferably a variable capacitancediode (varactor diode).

According to another preferred embodiment of the present invention, anelectronic apparatus includes the above-describedtemperature-compensated piezoelectric oscillator.

As described above, according to various preferred embodiments of thepresent invention, voltages that are variable depending upon thetemperature in opposite directions from each other are applied fromcorresponding voltage generation circuits to corresponding ends of thevariable capacitance element, which affects the oscillation frequency.Thus, a temperature-compensated piezoelectric oscillator that ensurestemperature compensation of an oscillation frequency and that outputs ahigh-frequency signal whose oscillation frequency does not depend uponthe temperature even when the power supply voltage is reduced isprovided.

In addition, according to preferred embodiments of the presentinvention, since each of the circuits for generating a temperaturecompensation voltage is defined by a simple analog circuit includingonly a thermistor and resistors, the temperature-compensatedpiezoelectric oscillator has a simple structure.

According to various preferred embodiments of the present invention,temperature compensation data corresponding to the ambient temperatureis stored in advance and is input to different DA converters to beconverted into voltage signals, and the voltage signals are applied toends of the variable capacitance element. Thus, atemperature-compensated piezoelectric oscillator that ensurestemperature compensation of an oscillation frequency and that outputs ahigh-frequency signal whose oscillation frequency does not depend on thetemperature even when the power supply voltage is reduced is provided.

Also, according to preferred embodiments of the present invention, byproviding the temperature-compensated piezoelectric oscillator, anelectronic apparatus which stably operates at a low power supply voltagewithout being affected by the ambient temperature and the operatingtemperature is provided.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram showing the structure of atemperature-compensated piezoelectric oscillator according to a firstpreferred embodiment of the present invention.

FIG. 2 includes a graph showing the temperature dependency of atemperature compensation output voltage (potential difference) of atemperature compensation voltage generation circuit in thetemperature-compensated piezoelectric oscillator shown in FIG. 1, agraph showing the temperature dependency of a temperature compensationoutput voltage of a temperature compensation voltage generation circuitin a known temperature-compensated piezoelectric oscillator, and anequivalent circuit diagram showing the temperature compensation voltagegeneration circuit in the known temperature-compensated piezoelectricoscillator.

FIG. 3 is an equivalent circuit diagram of a temperature-compensatedpiezoelectric oscillator according to a second preferred embodiment ofthe present invention.

FIG. 4 is an equivalent circuit diagram of a temperature-compensatedpiezoelectric oscillator according to a third preferred embodiment ofthe present invention.

FIG. 5 is a graph showing the temperature dependency of a temperaturecompensation output voltage (potential difference) of a temperaturecompensation voltage generation circuit in the temperature-compensatedpiezoelectric oscillator shown in FIG. 4.

FIG. 6 is an equivalent circuit diagram of a temperature-compensatedpiezoelectric oscillator according to a fourth preferred embodiment ofthe present invention.

FIG. 7 is a graph showing the applied voltage characteristics of thecapacitance of a varactor diode VD.

FIG. 8 is a graph showing the temperature dependency of a temperaturecompensation output voltage (potential difference) of a temperaturecompensation voltage generation circuit of the temperature-compensatedpiezoelectric oscillator shown in FIG. 6.

FIG. 9 is a block diagram showing a communication apparatus, which is anexample of an electronic apparatus according to a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A temperature-compensated piezoelectric oscillator according to a firstpreferred embodiment of the present invention will be described withreference to FIGS. 1 and 2.

FIG. 1 is an equivalent circuit diagram of the temperature-compensatedpiezoelectric oscillator according to the present preferred embodiment.

As shown in FIG. 1, the temperature-compensated piezoelectric oscillatorpreferably includes an AT-cut quartz crystal resonator (hereinafter,simply referred to as a “quartz crystal resonator”) XD, which is apiezoelectric element, an amplifying circuit 3 connected to one end ofthe quartz crystal resonator XD, a varactor diode VD, which is avariable capacitance element, connected the other end of the quartzcrystal resonator XD, and a temperature compensation voltage generationcircuit 10. Two outputs from the temperature compensation voltagegeneration circuit 10 are connected to ends of the varactor diode VD viaresistors R11 and R12, respectively.

The temperature compensation voltage generation circuit 10 includes afirst voltage generation circuit 1 connected to the resistor R11 and asecond voltage generation circuit 2 connected to the resistor R12. Eachof the first and second voltage generation circuits 1 and 2 is connectedto a power supply voltage (Vcc) terminal 4 and is grounded.

The first voltage generation circuit 1 includes a parallel circuitconnected to the Vcc terminal 4 and including a resistor R1 and athermistor TH1, which is a thermo-sensitive element, a resistor R3, anda thermistor TH3. The resistor R3 and the thermistor TH3 are connectedin series with the parallel circuit, and one end of the thermistor TH3is grounded. Also, the connection point of the resistor R3 and theparallel circuit, which includes the resistor R1 and the thermistor TH1,is connected to the cathode of the varactor diode VD via the resistorR1.

The second voltage generation circuit 2 includes a parallel circuitconnected to the Vcc terminal 4 and including a resistor R2 and athermistor TH2, which is a thermo-sensitive element, and a resistor R4.The resistor R4 is connected in series with the parallel circuit, andone end of the resistor R4 is grounded. Also, the connection point ofthe resistor R4 and the parallel circuit, which includes the resistor R2and the thermistor TH2, is connected to the anode of the varactor diodeVD via the resistor R12.

The anode of the varactor diode VD is connected to the quartz crystalresonator XD. The cathode of the varactor diode VD is grounded via ahigh-frequency by-pass capacitor C1.

The base of an NPN transistor Tr in the amplifying circuit 3 isconnected to the quartz crystal resonator XD, the collector of thetransistor Tr is connected to the Vcc terminal 4 via a resistor R22, andthe emitter of the transistor Tr is grounded via a resistor R23 and acapacitor C12. Also, a feedback capacitor C11 is connected between theemitter and the base of the transistor Tr. A resistor R21 for supplyinga bias current is connected between the base of the transistor Tr andthe Vcc terminal 4. Also, the collector of the transistor Tr isconnected to an output terminal 5 via a capacitor C14. Also, the Vccterminal 4 is RF-grounded via a capacitor C13. As a result, thetransistor Tr has a negative resistance at a resonant frequency of thequartz crystal resonator XD.

The first voltage generation circuit 1 of the temperature compensationvoltage generation circuit 10 applies a voltage signal that divides apower supply voltage Vcc with a division ratio between the parallelcircuit including the resistor R1 and the thermistor TH1 and the seriescircuit including the resistor R3 and the thermistor TH3 to the cathodeof the varactor diode VD via the resistor R11. In contrast, the secondvoltage generation circuit 2 applies a voltage signal that divides thepower supply voltage Vcc with a division ratio between the resistor R4and the parallel circuit including the resistor R2 and the thermistorTH2 to the anode of the varactor diode VD via the resistor R12.

The varactor diode VD functions as a capacitance element whosecapacitance changes in accordance with a potential difference betweenthe voltage from the second voltage generation circuit 2 and the voltagefrom the first voltage generation circuit 1.

An AT-cut quartz crystal resonator is preferably used as the quartzcrystal resonator XD. The resonant frequency of the quartz crystalresonator XD changes based on a cubic function with respect to theambient temperature. Also, the capacitance of the quartz crystalresonator XD, the capacitance of the varactor diode VD, and thecapacitance of the capacitor C1 define a resonant circuit. The quartzcrystal resonator XD resonates at a resonant frequency corresponding toa combined capacitance of these elements, together with the amplifyingcircuit 3.

The transistor Tr of the amplifying circuit 3 operates at the powersupply voltage Vcc, oscillates together with the above-mentionedresonant circuit, and outputs an oscillation signal to the outputterminal 5.

In accordance with a change in the ambient temperature, the voltagelevel of a voltage signal output from the first voltage generationcircuit 1 and the voltage level of a voltage signal output from thesecond voltage generation circuit 2 are changed.

FIG. 2( a) is a graph showing the temperature dependency of atemperature compensation output voltage (potential difference), that is,a voltage applied between the cathode and the anode of the varactordiode VD, of the temperature compensation voltage generation circuit inthe temperature-compensated piezoelectric oscillator shown in FIG. 1.FIG. 2( b) is a graph showing the temperature dependency of atemperature compensation output voltage, that is, a voltage appliedbetween the cathode and the anode of a varactor diode VD, of atemperature compensation voltage generation circuit in a knowntemperature-compensated piezoelectric oscillator. FIG. 2( c) is anequivalent circuit diagram of the temperature compensation voltagegeneration circuit in the known temperature-compensated piezoelectricoscillator. FIG. 2( a) shows simulation results in a case where, in FIG.1, the resistances of the resistors are set as: R1=30 kΩ, R2=20 kΩ, R3=1kΩ, and R4=1 kΩ, where the resistances of the thermistors at 25° C. areset as: TH1=2.31 kΩ, TH2=46.2 kΩ, and TH3=462 Ω, where the B constant ofeach of the thermistors is set to about between 3000 and 4000, and whereVcc is set to 3V. Also, FIG. 2( b) shows simulation results in a casewhere, in FIG. 2( c), the resistances of the resistors are set as:R01=30 kΩ, R02=10 kΩ, and R03=10 kΩ, where the resistances of thethermistors at 25° C. are set as: TH1=18.5 kΩ, TH2=1.24 kΩ, and TH3=201kΩ, where the B constant of each of the thermistors is preferably set tobetween 3000 and 4000, and where Vcc is set to 3V.

Accordingly, as shown in FIG. 2, even if the same power supply voltageVcc is supplied, the use of the temperature compensation voltagegeneration circuit according to this preferred embodiment increases therange of an output potential difference to about 1.2 V, as compared withthe range of the output potential difference, which is about 0.7 V, inthe known example. In other words, even if a power supply voltage Vcc isreduced, a reduction in the range of the voltage (potential difference)applied to a varactor diode is suppressed. This is because two outputvoltages from the temperature compensation voltage generation circuit 10vary in opposite directions from each other at least in a portion of atemperature range.

Since the varactor diode VD functions as a capacitance element whosecapacitance is in accordance with the above-described potentialdifference, a wider range of capacitance than the known example isachieved. In other words, the above-described structure of thetemperature compensation voltage generation circuit suppresses areduction in the range of the capacitance of the varactor diode VD evenwhen the power supply voltage is reduced.

Consequently, the combined capacitance of the resonant circuit includingthe quartz crystal resonator XD, the varactor diode VD, and thecapacitor C1 substantially changes, and the changed combined capacitanceoperates so as to substantially change the resonant frequency of theresonant circuit.

In contrast, since the quartz crystal resonator XD originally hastemperature dependency, the resonant frequency changes in accordancewith a change in the ambient temperature, as described above.

The resistors and the thermistors of the temperature compensationvoltage generation circuit are set in advance such that the amount ofchange in the resonant frequency due to the amount of change in thecapacitance of the varactor diode VD and the amount of change in theresonant frequency due to a change in the temperature of the quartzcrystal resonator XD compensate for each other. Thus, even if the powersupply voltage is reduced, the change in the resonant frequency issuppressed. In other words, a high-frequency signal having a stableoscillation frequency without depending on the ambient temperature isoutput.

A temperature-compensated piezoelectric oscillator according to a secondpreferred embodiment will be described with reference to FIG. 3. FIG. 3is an equivalent circuit diagram of the temperature-compensatedpiezoelectric oscillator according to this preferred embodiment.

As shown in FIG. 3, the temperature-compensated piezoelectric oscillatorpreferably includes a quartz crystal resonator XD, an amplifying circuit30 including the quartz crystal resonator XD, a varactor diode VD, whichis a variable capacitance element, connected to the quartz crystalresonator XD via a capacitor C32, and a temperature compensation voltagegeneration circuit 11. Two outputs from the temperature compensationvoltage generation circuit 11 are connected to ends of the varactordiode VD via low pass filters LPF34 and LPF35, respectively.

The temperature compensation voltage generation circuit 11 includes afirst DA converter 32 connected to the low pass filter LPF35 and asecond DA converter 33 connected to the low pass filter LPF34. Each ofthe first DA converter 32 and the second DA converter 33 is connected toa driving voltage (Vdd) terminal 4′. Also, each of the first DAconverter 32 and the second DA converter 33 is connected to atemperature compensation data controller 31 and is grounded (Vss).

The anode of the varactor diode VD is connected to the quartz crystalresonator XD of the amplifying circuit 30 via the capacitor C32 and isconnected to the low pass filter LPF34. The cathode of the varactordiode VD is grounded via a high-frequency by-pass capacitor C1 and isconnected to the low pass filter LPF35.

In the amplifying circuit 30, the quartz crystal resonator XD, aninverter 36, and a resistor R30 are connected in parallel with eachother. The parallel connection points are grounded via capacitors C33and C34, respectively. Also, an output side of the amplifying circuit 30(an output side of the inverter 36) is connected to an output terminal5. Here, the above-mentioned Vdd and Vss are used as power sources foran IC including the inverter 36.

The temperature compensation data controller 31 stores temperaturecompensation data corresponding to the ambient temperature in a memoryin advance. The temperature compensation data controller 31 reads thetemperature compensation data from the memory in accordance with atemperature detected by a temperature detection unit, and outputs thetemperature compensation data to the DA converter 32 and the DAconverter 33. The temperature compensation data determines a voltage(potential difference) to be applied to the ends of the varactor diodeVD in accordance with the temperature dependency of the resonantfrequency of the quartz crystal resonator XD in the amplifying circuit30, and data to be output to the DA converter 32 and data to be outputto the DA converter 33 are stored.

When the temperature compensation data controller 31 outputs temperaturecompensation data corresponding to a detected temperature to the DAconverter 32 and the DA converter 33 of the temperature compensationvoltage generation circuit 11, the DA converter 32 and the DA converter33 digital-to-analog convert the corresponding temperature compensationdata, and output the converted temperature compensation data as voltagesignals in an analog format. These voltage signals are applied to theends of the varactor diode VD via the low pass filters LPF34 and LPF35,respectively.

The capacitance of the varactor diode VD changes in accordance with adifference (potential difference) between the voltage signal from the DAconverter 33 and the voltage signal from the DA converter 32, andfunctions as a capacitance element.

An AT-cut crystal strip is preferably used as the quartz crystalresonator XD of the amplifying circuit 30. The resonant frequency of thequartz crystal resonator XD changes based on a cubic function withrespect to the ambient temperature. Since the resonant frequency isaffected by the capacitance of the varactor diode, changing thecapacitance in accordance with a temperature suppresses a variation inthe resonant frequency due to the ambient temperature. In other words,temperature compensation data stored in advance in the temperaturecompensation data controller 31 is set such that a variation in theresonant frequency of the quartz crystal resonator XD is suppressed bythe capacitance of the varactor diode VD in accordance with a detectedtemperature. Thus, a high-frequency signal having a constant oscillationfrequency without depending on the temperature is output.

Although the low pass filters LPF34 and LPF35 are provided at the outputsides of the DA converter 32 and the DA converter 33 in this preferredembodiment, the low pass filters LPF34 and LPF35 may be omitted.

A temperature-compensated piezoelectric oscillator according to a thirdpreferred embodiment will be described with reference to FIGS. 4 and 5.

FIG. 4 is an equivalent circuit diagram of the temperature-compensatedpiezoelectric oscillator according to this preferred embodiment.

The temperature-compensated piezoelectric oscillator shown in FIG. 4preferably has the same structure as the temperature-compensatedpiezoelectric oscillator shown in FIG. 1 with the exception that thefirst voltage generation circuit 1 of the temperature-compensatedpiezoelectric oscillator shown in FIG. 1 is replaced with a firstvoltage generation circuit 1′. In the first voltage generation circuit1′, the thermistor TH3 of the first voltage generation circuit 1 shownin FIG. 1 is omitted.

FIG. 5 is a graph showing the temperature dependency of a temperaturecompensation output voltage (potential difference) of the temperaturecompensation voltage generation circuit of the temperature-compensatedpiezoelectric oscillator shown in FIG. 4. FIG. 5 shows simulationresults in a case where, in FIG. 4, the resistances of the resistors areset as: R1=50 kΩ, R2=100 kΩ, R3=20 kΩ, and R4=1 kΩ, where theresistances of the thermistors at 25° C. are set as: TH1=2.31 kΩ andTH2=46.2 kΩ, where the B constant of each of the thermistors is set tobetween about 3000 and about 4000, and where Vcc is set to 3V.

Accordingly, the circuit structure shown in FIG. 4 allows a voltage(potential difference) applied to the varactor diode VD to be in a curvethat approximately corresponds to the cubic function. Thus, almost allof the variation in the resonant frequency of the quartz crystalresonator is compensated for.

Consequently, a temperature-compensated piezoelectric oscillator thatoutputs a high-frequency signal having an approximately constantoscillation frequency without depending on the ambient temperature canbe provided with a simpler structure.

A temperature-compensated piezoelectric oscillator according to a fourthpreferred embodiment will be described with reference to FIGS. 6 to 8.

FIG. 6 is an equivalent circuit diagram of the temperature-compensatedpiezoelectric oscillator according to this preferred embodiment.

The temperature-compensated piezoelectric oscillator shown in FIG. 6preferably has the same structure as the temperature-compensatedpiezoelectric oscillator shown in FIG. 4 with the exception that thesecond voltage generation circuit 2 of the temperature-compensatedpiezoelectric oscillator shown in FIG. 4 is replaced with a secondvoltage generation circuit 2′. In the second voltage generation circuit2′, the Vcc terminal 4 is connected to the resistor R4, the resistor R4is connected to the parallel circuit including the thermistor TH2 andthe resistor R2, and one end of the parallel circuit is grounded. Also,the connection point of the resistor R4 and the parallel circuit isconnected to the varactor diode VD via the resistor R12.

With this structure, due to the combination of settings of the elementvalues (impedances) of the elements (the resistors R1 to R4 and thethermistors TH1 and TH2) and the B constants of the thermistors in atemperature compensation voltage generation circuit 13, a negativevoltage (a forward bias voltage in terms of the diode) can be applied tothe varactor diode VD.

FIG. 7 is a graph showing the applied voltage characteristics of thecapacitance of the varactor diode VD. In this graph, the forwarddirection of the applied voltage represents a negative direction interms of the diode characteristics. As shown in this graph, thecapacitance of the varactor diode VD increases as the applied voltage isreduced to a negative voltage. This capacitance increases until itreaches a voltage Vf at which a current starts to flow in the diode.

FIG. 8 is a graph showing the temperature dependency of a temperaturecompensation output voltage (potential difference) of the temperaturecompensation voltage generation circuit of the temperature-compensatedpiezoelectric oscillator shown in FIG. 6. FIG. 8 shows simulationresults in a case where, in FIG. 6, the resistances of the resistors areset as: R1=50 kΩ, R2=20 kΩ, R3=20 kΩ, and R4=20 kΩ, where theresistances of the thermistors at 25° C. are set as: TH1=23.1 kΩ andTH2=37.0 kΩ, where the B constant of each of the thermistors is set tobetween about 3000 and about 4000, and where Vcc is set to 3V.

Accordingly, the circuit structure shown in FIG. 6 allows a voltage(potential difference) applied to the varactor diode VD to be in a curvethat approximately corresponds to the cubic function, and increases therange of the voltage (potential difference) applied to the varactordiode VD. Thus, almost all of the variation in the resonant frequency iscompensated for even if a quartz crystal resonator highly dependent onthe temperature, that is, a quartz crystal resonator whose resonantfrequency greatly changes depending on the ambient temperature is used.

Consequently, a temperature-compensated piezoelectric oscillator thatoutputs a high-frequency signal having an approximately constantoscillation frequency without depending on the ambient temperature isprovided.

Although a temperature-compensated piezoelectric oscillator including aColpitts oscillation circuit and an inverter oscillation circuit hasbeen explained in each of the foregoing preferred embodiments, similaradvantages can be achieved by using an oscillation circuit, such as aHartley oscillation circuit, a Pierce oscillation circuit, or a Clapposcillation circuit. Also, although the oscillation circuit including abipolar transistor has been explained, a field-effect transistor may beused. In addition, similar advantages can be achieved by using anoscillation circuit including a logic element, such as a C-MOS. Also,similar advantages can be achieved by inserting circuit elements, suchas a capacitor and an inductor, in the temperature compensation voltagegeneration circuit shown in each of the foregoing preferred embodiments.Also, the piezoelectric element is not limited to a quartz crystalresonator. Similar advantages can be achieved by using a surfaceacoustic wave resonator, a ceramic resonator using bulk resonance, alithium tantalate resonator, or lithium niobate resonator.

An electronic apparatus according to a fifth preferred embodiment willbe described with reference to FIG. 9.

FIG. 9 is a block diagram showing a communication apparatus, which is anexample of an electronic apparatus.

As shown in FIG. 9, a communication apparatus 90 includes an antenna901, a duplexer 902, amplifiers 903 a and 903 b, mixers 904 a and 904 b,a voltage control oscillator 905, a PLL circuit 906, a low pass filter907, a temperature-compensated piezoelectric oscillator 910 according tothe present invention, a modulator Tx, and a demodulator Rx.

The PLL circuit 906 receives an output signal from the voltage controloscillator 905, compares the phase of the output signal with a divisionsignal of an oscillation signal of the temperature-compensatedpiezoelectric oscillator 910, and outputs a control voltage such thatthe voltage control oscillator 905 has a predetermined frequency.

The voltage control oscillator 905 receives the control voltage at acontrol terminal via the low pass filter 907, and outputs ahigh-frequency signal corresponding to the control voltage. Thehigh-frequency signal is given to each of the mixers 904 a and 904 b asa local oscillation signal.

The mixer 904 a mixes an intermediate frequency signal and a localoscillation signal output from the modulator Tx, and converts the mixedsignal into a transmission signal. The transmission signal is amplifiedby the amplifier 903 a, and is emitted from the antenna 901 via theduplexer 902.

The reception signal received at the antenna 901 is amplified by theamplifier 903 b via the duplexer 902. The mixer 904 b mixes thereception signal amplified by the amplifier 903 b and the localoscillation signal from the voltage control oscillator 905, and convertsthe mixed signal into an intermediate frequency signal. The intermediatefrequency signal is detected by the demodulator Rx.

As described above, the use of the temperature-compensated piezoelectricoscillator 910 shown in each of the foregoing preferred embodimentsachieves a compact communication apparatus having excellentcommunication characteristics. Although the communication apparatus 90has been explained as an electronic apparatus including thetemperature-compensated piezoelectric oscillator according to variouspreferred embodiments of the present invention, the electronic apparatusaccording to the present invention is not limited to a communicationapparatus.

While the present invention has been described with respect to preferredembodiments, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than those specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

1. A temperature-compensated piezoelectric oscillator comprising: a piezoelectric element: an amplifying circuit connected to a first end of the piezoelectric element; a variable capacitance element connected a second end of the piezoelectric element; a compensation voltage generation circuit for applying a voltage corresponding to a temperature to the variable capacitance element; and a temperature compensation data generation circuit for detecting the ambient temperature and for generating temperature compensation data corresponding to the detected temperature; wherein the compensation voltage generation circuit includes a first voltage generation circuit for appling to a first end of the variable capacitance element a first voltage that is variable depending on an ambient temperature and second voltage generation circuit for applying to a second end of the variable capacitance element a second voltage that is variable depending on the ambient temperature in a direction opposite to the first voltage; and each of the first and second voltage generation circuits includes a DA converter arranged to convert the temperature compensation data in a digital format into an analog signal.
 2. The temperature-compensated piezoelectric oscillator according to claim 1, wherein the piezoelectric element is an AT-cut quartz crystal resonator.
 3. The temperature-compensated piezoelectric oscillator according to claim 1, wherein the variable capacitance element is a varactor diode.
 4. A temperature-compensated piezoelectric oscillator comprising: a piezoelectric element; an ammplifying circuit connected to a first end of the piezoelectric element; a variable capacitance element connected a second end of the piezoelectric element; and a compensation voltage generation circuit for applying a voltage corresponding to a temperature to the variable capacitance element; wherein the compensation voltage cieneration circuit includes a first voltage generation circuit for applying to a first end of the variable capacitance element a first voltage that is variable depending on an ambient temperature and second voltacie cieneration circuit for applying to a second end of the variable capacitance element a second voltage that is variable deDendinQ on the ambient temDerature in a direction opposite to the first voltage: the variable capacitance element is a varactor diode; and an anode of the varactor diode is connected to the piezoelectric element and a cathode of the varactor diode is grounded via a high-frequency bypass capacitor.
 5. A temperature-compensated piezoelectric oscillator comprising: a piezoelectric element: an amplified circuit connected to a first end of the piezoelectric element a variable capacitance element connected a second end of the piezoelectric element; and a compensation voltage generation circuit for appplying a voltage corresponding to a temperature to the variable capacitance element; wherein the copensation voltage generation circuit includes a first voltage generation circuit for applying to a first end of the variable capacitance element a first voltage that is variable depending on an ambient temperature and second voltage generation circuit for applying to a second end of the variable capacitance element a second voltage that is variable depending on the ambient temperature in a direction opposite to the first voltage; and the amplifying circuit includes an NPN transistor, a plurality of resistors and at least one capacitor.
 6. The temperature-compensated piezoelectric oscillator according to claim 5, wherein a base of the NPN transistor is connected to the piezoelectric element, a collector of the NPN transistor is connected a terminal of the temperature-compensated piezoelectric oscillator via one of the plurality of resistors, and an emitter of the NPN transistor is grounded via another of the plurality of resistors and the at least one capacitor.
 7. An electronic apparatus comprising the temperature-compensated piezoelectric oscillator as set forth in claim
 1. 8. An electronic apparatus comprising: a temperature-compensated piezoelectric oscillator comprising: a piezoelectric element; an amplifying circuit connected to a first end of the piezoelectric element: a variable capacitance element connected a second end of the piezoelectric element; and a compensation voltage generation circuit for applying a voltage corresponding to a temperature to the variable capacitance element; wherein the compensation voltage generation circuit includes a first voltage generation circuit for applying to a first end of the variable capacitance element a first voltage that is variable depending on an ambient temperature and second voltage generation circuit for applying to a second end of the variable capacitance element a second voltage that is variable dependincg on the ambient temperature in a direction opposite to the first voltage; an antenna; a duplexer connected to the antenna; a plurality of amplifiers connected to the duplexer; a plurality of mixers, each being connected to a respective one of the plurality of amplifiers; a voltage control oscillator connected to said plurality of mixers; and a PLL circuit and a low pass filter connected to the voltage control oscillator; wherein said temperature-compensated piezoelectric oscillator is connected to said the voltage control oscillator.
 9. The temperature-compensated piezoelectric oscillator according to claim 4, wherein each of the first and second voltage generation circuits includes at least one thermo-sensitive element and a plurality of resistance elements.
 10. The temperature-compensated piezoelectric oscillator according to claim 4, wherein each of the first and second voltage generation circuits includes a parallel circuit connected to a terminal of the temperature-compensated piezoelectric oscillator and includes a first thermo-sensitive element and a first resistance element, a second thermo-sensitive element and a second resistance element connected in series to said parallel circuit, one end of said second thermo-sensitive element being grounded.
 11. The temperature-compensated piezoelectric oscillator according to claim 4, wherein each of the first and second voltage generation circuits includes a thermo-sensitive element and a first resistance element connected in parallel to define a parallel circuit and a second resistance element connected in series to the parallel circuit.
 12. The temperature-compensated piezoelectric oscillator according to claim 11, wherein the thermo-sensitive element is a thermistor.
 13. The temperature-compensated piezoelectric oscillator according to claim 4, wherein the piezoelectric element is an AT-cut quartz crystal resonator.
 14. The temperature-compensated piezoelectric oscillator according to claim 5, wherein each of the first and second voltage generation circuits includes at least one thermo-sensitive element and a plurality of resistance elements.
 15. The temperature-compensated piezoelectric oscillator according to claim 5, wherein each of the first and second voltage generation circuits includes a parallel circuit connected to a terminal of the temperature-compensated piezoelectric oscillator and includes a first thermo-sensitive element and a first resistance element, a second thermo-sensitive element and a second resistance element connected in series to said parallel circuit, one end of said second thermo-sensitive element being grounded.
 16. The temperature-compensated piezoelectric oscillator according to claim 5, wherein each of the first and second voltage generation circuits includes a thermo-sensitive element and a first resistance element connected in parallel to define a parallel circuit and a second resistance element connected in series to the parallel circuit.
 17. The temperature-compensated piezoelectric oscillator according to claim 16, wherein the thermo-sensitive element is a thermistor.
 18. The temperature-compensated piezoelectric oscillator according to claim 5, wherein the piezoelectric element is an AT-cut quartz crystal resonator.
 19. The temperature-compensated piezoelectric oscillator according to claim 5, wherein the variable capacitance element is a varactor diode.
 20. The temperature-compensated piezoelectric oscillator according to claim 1, wherein each of the first and second voltage generation circuits includes at least one thermo-sensitive element and a plurality of resistance elements.
 21. The temperature-compensated piezoelectric oscillator according to claim 1, wherein each of the first and second voltage generation circuits includes a parallel circuit connected to a terminal of the temperature-compensated piezoelectric oscillator and includes a first thermo-sensitive element and a first resistance element, a second thermo-sensitive element and a second resistance element connected in series to said parallel circuit, one end of said second thermo-sensitive element being grounded.
 22. The temperature-compensated piezoelectric oscillator according to claim 1, wherein each of the first and second voltage generation circuits includes a thermo-sensitive element and a first resistance element connected in parallel to define a parallel circuit and a second resistance element connected in series to the parallel circuit.
 23. The temperature-compensated piezoelectric oscillator according to claim 20, wherein the thermo-sensitive element is a thermistor. 